Place Mosquito Dunks in ponds or standing water and it's active ingredient, Bacillus thuringiensis subspecies israaelensescalled BTI kills mosquito and black fly larvae for 30 days or more. While floating they slowly release a long term biological mosquito larvacide killing the larvae before they hatch into adults. Will not effect fish, plants, people or wildlife. Alternative wetting and drying will not reduce it's effectiveness. Use one dunk per 100 sqft of water surface.